As you have discovered Xert is a dual device platform comprised of a phone app to control the workout plus a browser page (PC/laptop/tablet) to view the workout being run on the phone.
All control of the workout is through the EBC phone app.
On the Android app you can swipe up to stop a workout in progress or tap the standard Android back button.
Quick Start Guide: EBC – Xert (baronbiosys.com)
If you prefer a simpler experience for the browser page view, you can close the Session Player and run the Remote Player instead (traditional blue block chart).
Closing either browser player has no effect on the workout running on the phone app.
No need to schedule a workout unless you plan to start it at that time. Scheduling is most likely to be used when you plan to have other Xert users join you for the session (public or private with team/club members). When that occurs the XBL podcast screen is replaced with a YT video box and group chat is active. The workout stats box is replaced with a leaderboard (non-competitive).
Many of us select a workout (Play Now) right before we workout. When you do that the workout doesn’t actually start until you tap Start on the phone app.
Beginner’s Guide: Getting Started With Xert – Xert (baronbiosys.com)
Lots of newbie tips posted here –
Beginner questions - Support - Xert Community Forum (xertonline.com)
Training Right For Your Event – Xert (baronbiosys.com)
FAQs here – Support Home – Xert
Academy series videos are the quickest way to get up to speed. Here they are in order (building blocks) – https://tinyurl.com/yfrucamh
This does means investing 3+ hours of your time to learn something new and different, but it’s well worth it IMO/IME. Xert is tailor-made for those of us who like to self-coach.
You can alternately view the series as three one-hour low intensity endurance workouts by selecting them from the Session Library. Seach for “mastering”. The order is Discover, Improve, Perform.
Now that I know how everything works, I am up and running my selected workout for the day in less than a minute. Or I’m headed out the door for a free ride to a focus duration which could be an Easy Polar Endurance ride (think constant Z2) or a Difficult Mixed Puncheur Ride (intermittent intervals targeting 4:00 duration point on power curve).
Xert is different on purpose. 
XSS is not TSS.
TP is not FTP.
Workouts are not limited to %FTP blocks.
The Planner it not a calendar to be filled in.
The rainbow gauge tells all. 
Hang in there. The aha moments are coming.
The 30-day trial includes access to a limited set of workouts, and you can only load 90 days of historical data. When you subscribe, the workout library expands to 500+ and you can load as much history as you want. You can also import your favorite workouts sourced elsewhere and optionally improve them during import (convert sections to SMART intervals which scale to your fitness signature).